The http://notation.com/UpgradeOrder.php upgrade order page is confusing. I'll fix that. In Step 4, choose to upgrade _from_ the copy of Notation Musician 1.0 that you purchased rather than from the free upgrade you later received (Musician 2.0):

We'll fix this upgrade order page so that it will no longer list the free upgrades, which do not themselves qualify for further upgrades, purchased or free. Only the purchased copies qualify for upgrades, purchased or free.